Our present invention relates to a method of and an apparatus for thin film deposition on a substrate and especially for applying an optical coating to a substrate with low absorption. More particularly, the invention relates to the application of thin film coatings by reactive deposition.
A thin film coating method and apparatus are known from Swiss patent CH 928/85 and utilize an ion plating technique whereby material vaporization at least in part by an electron beam gun is ionized in a reactive environment formed by at least one gas admitted to the evacuatable reaction chamber and a resulting reaction production is thereby deposited on the substrate. The system utilizes low energy electron bombardment of the component in a crucible. The substrates tend to be electrically floating and are polarized under the conditions described in that patent at a bias voltage between xe2x88x9205 and xe2x88x9260 volts.
Experience with that system has shown that when efforts are made to deposit optical coatings, i.e. coatings with a relatively high refractive index, the coatings are at least partially absorbing. This drawback has limited the applicability of the coating method described in the Swiss patent and the optical losses which are associated with the absorption resulting from the coating has greatly limited the applicability of the method. This is especially the case for multilayer coatings. Indeed, it is impossible to use such coatings when a high laser damaged threshold is required for the product.
The absorption appears to be a result of the energetic ion bombardment of the material in the crucible and other ions present in the process atmosphere.
The method did permit highly compact films to be obtained even on substrates at low temperatures and with refractive indices of the coating which is close to the refractive index of the bulk material. The method also gave rise to coating layers which had excellent mechanical and environmental characteristics.
It is the principal object of the present invention to provide an improved method of coating a substrate with a thin film, especially an optical film, whereby drawbacks of the earlier systems are avoided.
It is another object of this invention to provide a thin film coating method whereby optical coatings of low absorption can be formed economically and reliably and with excellent mechanical and environmental properties.
It is also an object of this invention to provide an improved apparatus for applying thin film coatings to substrates.
These objects can be attained, in accordance with the invention, in a method which comprises the steps of:
(a) mounting a substrate to be coated on a substrate holder in an evacuatable chamber and so that the substrate is spacedly juxtaposed with a crucible containing a component of a coating to be applied to the substrate;
(b) evacuating the chamber;
(c) positioning a shutter between the crucible and the substrate and heating the component in the crucible with a high energy beam;
(d) admitting a gas mixture to the chamber;
(e) connecting the substrate holder to a radio frequency or pulsed direct current source so that the substrate holder is poled cathodic and a plasma is formed at least around the substrate to create a self bias of several hundreds of volts on the substrate holder and a surface of the substrate is bombarded with particles from the plasma;
(f) withdrawing the shutter from its position between the crucible and the substrate, bombarding the component with low energy electrons to ionize the component at least in part and depositing the component and the at least one gas on the substrate; and
(g) controlling the ionization of the component so that the self bias is reduced by at least 50%.
Advantageously, the coating is carried out in a reactive mode, i.e. with the ionized components reacting with at least one gas in the gas mixture. The coatings which can be deposited can be oxides, nitrides, oxynitrides and carbides of substantially any component, usually a metal which can be melted and ionized in the crucible, including aluminum, tin, silver and chromium. Elemental metals may be deposited without reaction, for example, gold.
The apparatus which is used can comprise:
a vacuum chamber connectable to a pump adapted to evacuate the chamber;
at least one crucible in the chamber;
a substrate holder in the chamber receiving a substrate to be coated and juxtaposed with the crucible;
a mechanical shutter in the chamber interposable between the crucible and the substrate;
a high-energy source for heating a component of a coating to be deposited upon the substrate in the crucible;
a radio frequency or pulsed direct current source connectable to the substrate holder for producing a plasma around the substrate and imparting a self-bias to the substrate holder poling the substrate holder cathodic;
means for feeding a gas mixture to the chamber including at least one gas reactive with the component to form a coating on the substrate; and
a low energy electron source for ionizing the component to reduce the self-bias and deposit a reaction product of the component and the at least one gas on the substrate, the shutter being movable from between the crucible and the substrate to permit ionization of the component.
The invention utilizes the plasma generated around the substrate and at the substrate holder by the radio frequency or pulsed direct current source connected to the cathodically poled substrate holder to produce a self-bias around and in front of the substrate or substrates which can vary from several tens of volts to many hundreds of volts and usually is at least several hundred volts. The gas which is used can include an inert gas like argon and reactive gases like oxygen and nitrogen or mixtures thereof and the plasma may be maintained at a pressure in the chamber of 10xe2x88x923 to 10xe2x88x922 Torr. The chamber is initially evacuated to about say 10xe2x88x926 Torr before the gas mixture is admitted to the chamber.
The effects of the plasma are manifold and include:
ion bombardment from the plasma of the substrate surface usually by reactive gas molecules which appears to activate the surface and promote bonding of the thin film coating and mechanical integrity of the latter. The plasma also contributes to ionization and excitation of the reactive gas which tends to drive the reaction in the direction of formation of the reaction products and compensates for possible disassociation caused by energetic ion bombardment.
In addition, the plasma appears to bring about an ion bombardment of the growing film and hence a higher film compactness during its growth with a consequent improvement in adherence of the film to the substrate.
The plasma also contributes to an increase in the duration of the xe2x80x9cadatomxe2x80x9d phase which also results in an increase in compactness and mechanical stability of the film and a refractive index value which is close to that of the bulk material.
Finally it has been found, most surprisingly, that the presence of the plasma, coupled with the dramatic decrease in the self-bias brought about by ionization of the coating components, results in an increase in the maximum deposition rate without significant absorption so that the duration of the process can be reduced along with the processing cost.
The apparatus of the invention permits optical coatings to be applied to substrates without noticeable absorption of the film and a significant increase in the refractive index by comparison with the reactive indices of coatings applied with other apparatus even at higher deposition rates.